EP3505490B1 - A method for forming a qubit device - Google Patents

A method for forming a qubit device Download PDF

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Publication number
EP3505490B1
EP3505490B1 EP17211120.5A EP17211120A EP3505490B1 EP 3505490 B1 EP3505490 B1 EP 3505490B1 EP 17211120 A EP17211120 A EP 17211120A EP 3505490 B1 EP3505490 B1 EP 3505490B1
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Prior art keywords
insulating layer
gate electrode
forming
substrate
contact
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German (de)
French (fr)
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EP3505490A1 (en
Inventor
Clement Merckling
Nadine Collaert
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Interuniversitair Microelektronica Centrum vzw IMEC
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Interuniversitair Microelektronica Centrum vzw IMEC
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Priority to EP17211120.5A priority Critical patent/EP3505490B1/en
Priority to US16/222,911 priority patent/US10930750B2/en
Priority to CN201811590556.5A priority patent/CN110009107B/en
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Definitions

  • the present inventive concept relates to a method for forming a qubit device.
  • the present inventive concept further relates to a qubit device.
  • Quantum information science has the potential to radically improve existing techniques and devices for sensing, computation, simulation, and communication.
  • a major challenge in quantum information processing systems is to realize qubits with a sufficient degree of coherence while still allowing manipulation and measurements.
  • One type of qubit device showing promise are devices based on so-called Majorana fermions.
  • a Majorana fermion is its own antiparticle and may in a quantum mechanical framework be described as a superposition of an electron and a hole.
  • WO 2016/000836 A1 discloses semiconductor based Josephson junctions and their applications within the field of quantum computing, in particular a tuneable Josephson junction device being used to construct a gateable transmon qubit.
  • the reference discloses a Josephson junction comprising an elongated hybrid nanostructure comprising superconductor and semiconductor materials and a weak link, wherein the semiconductor weak link is formed by a semiconductor segment of the elongated hybrid nanostructure where the superconductor material has been removed.
  • An elongated hybrid nanostructure is liberated from their growth substrate by a brief sonication and a small amount of the resulting suspension can subsequently be deposited on doped Si substrates capped with 500 nm SiO 2 .
  • Roddare S et al discloses in "InAs nanowire hot-electron Josephson transistor" (ARXIV.ORG, CORNELL UNIVERSITY LIBRARY, 201 OLIN LIBRARY CORNELL UNIVERSITY ITHACA, NY 14853, 10 March 2010, DOI: 10.1007/S12274-010-0077-6 ) an InAs NW Josephson transistor where supercurrent is controlled by hot-quasiparticle injection from normalmetal electrodes.
  • An objective of the present inventive concept is to address this issue in the prior art. Further and alternative objectives may be understood from the following.
  • the inventive method enables accurate, repeatable and scalable manufacturing of qubit devices.
  • the combination of a group III-V semiconductor channel structure and superconductor source and drain contacts enable Majorana fermion based qubit devices.
  • qubit device is hereby meant a semiconductor device with a configuration such that, under the correct operating conditions, it may support or provide states or particles (or more specifically quasiparticles in the case of Majorana fermions) which may operate as qubits whose state may be manipulated and detected.
  • the channel structure is formed by patterning an epitaxially grown group III-V material structure the typical prior art approach of growing nanostructures (such as vertical nanowires) on a separate wafer and thereafter transferring them to a target substrate may be avoided.
  • the method thereby allows an overall more efficient process as transfer of nanowires to intended positions at a target substrate may be delicate and difficult to scale.
  • the forming of the channel structure by patterning allows a considerable degree of control of the dimension, shape and arrangement of the channel structure in relation to the gate electrode.
  • the gate electrode By forming the gate electrode as an embedded gate electrode, exposure of the channel structure to the potentially adverse process conditions of gate electrode and gate dielectric formation may be avoided. Moreover, the embedded configuration of the gate electrode enables a gate-channel coupling of a strength suitable for qubit device operation to be achieved.
  • the method enables scalable fabrication of qubit devices with an improved uniformity in terms of structure and performance.
  • the superconductor source contact and the superconductor drain contact are advantageously formed subsequent to forming the channel structure. More specifically, the method may comprise forming the superconductor source contact and the superconductor drain contact on the channel structure at mutually opposite sides of said portion of the gate electrode. This facilitates achieving a proper alignment between the source/drain portions of the channel structure and the source/drain contacts.
  • the upper surface of the substrate is intended a main surface of the substrate on which the processing according to the inventive method is performed.
  • "upper” should not be construed to require a particular orientation of the substrate.
  • first horizontal direction refers to a direction along the substrate, i.e. along the upper surface of the substrate.
  • second horizontal direction refers to a direction along the substrate, which second horizontal direction is different from the first horizontal direction.
  • the first and second horizontal directions may represent mutually perpendicular directions but may more generally extend at an angle with respect to each other.
  • horizontal plane may be used to refer to a plane defined by the first and second horizontal directions. In other words, the term horizontal plane may denote a plane parallel to the upper (main) surface of the substrate.
  • a vertical direction may refer to a direction which is normal to the upper (main) surface of the substrate, or perpendicular to the first and second horizontal directions.
  • a superconductor is hereby meant any material exhibiting superconductivity when kept at a temperature below a critical temperature Tc, specific to the material.
  • Tc of Al is 1.20 K
  • Tc of Ta is 4.48 K
  • T C of Ti is 0.39 K
  • T C of Nd is 9.26 K.
  • the mask is formed such that the substrate contact part is exposed by the mask. Accordingly the substrate contact part is removed during the etching wherein the channel structure is electrically insulated from the substrate by the insulator layer.
  • said forming of the gate electrode embedded in the insulating layer comprises:
  • the gate electrode is formed to be elongated in the first horizontal direction.
  • An elongated gate electrode enables definition of a short gate-channel interface, along the direction of the channel. A highly localized gate control may thereby be achieved.
  • the method may further comprise forming a pair of gate contacts on the gate electrode, at mutually opposite sides of the channel structure . Easy accessible electrical contact to the gate electrode may thereby be provided.
  • the contacts may be formed spaced apart from the channel structure to mitigate undesired stray capacitive coupling between the contacts and the channel.
  • said gate electrode forms a first gate electrode and the method further comprises forming at least a second gate electrode, each one of the gate electrodes being embedded in the insulating layer and extending in said first direction. Providing more than gate electrode enables improved channel control along the length of the channel structure.
  • the semiconductor disc part may be formed to overlap a respective portion of each one of the gate electrodes, wherein the channel structure may extend across each one of said portions of the gate electrodes. Gate electrode-channel structure overlap may thereby be obtained at plural positions in an efficient manner.
  • the epitaxial growth process may be adapted such that a growth rate in a horizontal plane is greater than a growth rate in a vertical direction during a part of the epitaxial growth process in which the disc part is grown. Thereby a disc part having a greater horizontal dimension than vertical dimension may be grown. This in turn enables forming of a relatively long and thin channel structure.
  • the channel structure is formed to be elongated in the second horizontal direction, the second horizontal direction being different from the first horizontal direction.
  • a channel structure having a high length to thickness ratio and, optionally, a high width to thickness ratio may be achieved.
  • Such a channel structure may be referred to as a nanowire.
  • the disc part may be formed to enclose the upper portion of the substrate contact part in a horizontal plane.
  • the channel structure may hence be formed to extend on either side of the substrate contact part.
  • Superconductor contacts may thereby be conveniently formed wherein portions of the channel structure covered by the contact mask (i.e. portions other than the source portion and the drain portion) may be protected, at least to some extent, from the process conditions of the contact formation.
  • the forming of the source and drain superconductor contacts in the respective mask openings may include deposition of a superconductor material in the respective openings.
  • a superconductor seed layer may be formed on portions of the channel structure exposed in the contact openings. This may improve an interface quality between the superconductor source/drain contacts and the channel structure.
  • the superconductor source contact and the superconductor drain contact may include, or consist of, Al, Ta, Ti or Nd.
  • the epitaxial growth process may comprise growing the semiconductor structure of InSb, InAs, InGaAs or InGaSb. These semiconductor materials, when used in combination with superconductor contacts, enable formation of Majorana fermions in the device channel.
  • At least the disc part may be formed of monocrystalline InSb, InAs, InGaAs or InGaSb.
  • FIG. 1-13 Each one of the figures shows a cross-sectional side view (figure a) of the structure and a top down view (figure b).
  • the cross-sectional view in figure A is taken along the geometrical line indicated in the correspondingly numbered figure B (i.e. line AA' or BB').
  • the axis Z denotes a vertical direction, corresponding to a normal direction with respect to an upper surface 100a of a substrate 100.
  • the axes X and Y refer to mutually orthogonal first and second horizontal directions, i.e. directions being parallel to the main plane of extension of the substrate 100 (or correspondingly the upper surface 100a thereof).
  • the relative dimensions of the shown elements such as the relative thickness of the layers of structures, is merely schematic and may, for the purpose of illustrational clarity, differ from a physical structure.
  • FIGS 1A, 1B illustrate a semiconductor substrate 100.
  • the semiconductor substrate 100 may be single material substrate or be formed of a combination of different materials, such as of various layers stacked on top of each other.
  • at least an upper surface 100a of the substrate 100 is formed of an elemental group IV semiconductor or compound group IV semiconductor.
  • the upper surface may be formed by a [111] face of silicon (Si).
  • Possible substrate 100 structures include a Si substrate or a silicon-on-insulator (SOI) substrate.
  • the upper surface 100a of the substrate 100 may be referred to as a front side surface of the substrate 100.
  • a first partial insulating layer 102a has been formed on the upper surface 100a of the substrate 100.
  • the first partial insulating layer 102a covers the upper surface 100a.
  • the first partial insulating layer 102a may be an oxide layer, for instance a silicon oxide such as SiO 2 .
  • the first partial insulating layer 102a may also be a dielectric layer of for instance a high-K dielectric material such as aluminum oxide or some other CMOS compatible gate dielectric.
  • the first partial insulating layer 102a may be deposited on the surface 100a by any suitable and conventional deposition technique, such as chemical vapor deposition (CVD) or atomic layer deposition (ALD), or by thermal oxidation.
  • CVD chemical vapor deposition
  • ALD atomic layer deposition
  • a trench 104 extending partially through the first partial insulating layer 102a has been formed.
  • a longitudinal dimension of the trench 104 extends in the first horizontal direction X.
  • a vertical dimension of the trench 104 extends in the vertical direction Z.
  • a bottom surface of the trench 104 is formed by a remaining thickness portion of the first partial insulating layer 102a.
  • a trench mask layer 103 has been formed on the first partial insulating layer 102a.
  • a trench defining opening has been formed in the mask layer 103. The opening has subsequently been transferred into the first partial insulating layer 102a by etching of the material of the first partial insulating layer 102a to form the trench 104.
  • the mask layer 103 may be a photo-resist based mask layer 103 of any other typical lithographic stack compatible with the material forming the first partial insulating layer 102a. Any conventional wet or dry etching process allowing etching of the material of the first partial insulating layer 102a may be employed. Subsequent to etching the trench 104, the mask layer 103 may be removed.
  • the trench 104 may by way of example be formed with a width in the range of 20-100 nm, depending among others on the intended length of the final channel structure which is to be formed.
  • a gate electrode 106 has been formed in the trench 104.
  • the gate electrode 106 is elongated and extends along the substrate upper surface, in the first horizontal direction X.
  • the gate electrode 106 is electrically insulated from the substrate 100 by the insulating layer 102.
  • a conductive gate electrode material may be deposited in the trench 104.
  • the conductive gate electrode material may be a metal or a metal alloy. However other gate electrode materials are also possible, such as polysilicon.
  • the material may be deposited by any conventional deposition technique, such as by ALD, CVD or physical vapor deposition (PVD).
  • the material may be deposited to fill the trench 104 and cover the first partial insulating layer 102a.
  • Overburden gate electrode material i.e. material portions deposited outside of the trench 104) may subsequently be removed from outside of the trench by chemical mechanical polishing (CMP) and/or an etch back process, thereby exposing an upper surface of the insulating layer 102a.
  • CMP
  • the gate electrode 106 and the first partial insulating layer 102a has been covered by a second partial insulating layer 102b.
  • the first partial insulating layer 102a and the second partial insulating layer 102b together form a compound insulating layer, hereinafter referred to as the insulating layer 102, embedding the gate electrode 106.
  • the second partial insulating layer 102b may be formed of a same material as the first partial insulating layer 102a.
  • the second partial insulating layer 102b may be deposited in a similar manner as the first partial insulating layer 102a.
  • a total thickness of the insulating layer 102 may for instance be in the range of 10-50 nm.
  • an aperture 108 has been formed in the insulating layer 102.
  • the aperture 108 exposes an upper surface portion 100b (i.e. a portion of the upper surface 100a) of the substrate 100.
  • the aperture 108 extends vertically through the insulating layer 102.
  • An aperture mask layer 107 has been formed on the insulating layer 102.
  • An aperture defining opening has been formed in the mask layer 107.
  • the opening has subsequently been transferred into the insulating layer 102 by etching of the material (of the insulating layer 102 to form the aperture 108.
  • the mask layer 107 may be a photo-resist based mask layer 107 of any other typical lithographic stack compatible with the material forming the first partial insulating layer 102a. Any conventional wet or dry etching process allowing etching of the material of the insulating layer 102 may be employed. Subsequent to etching the aperture 108 the mask layer 107 may be removed.
  • a semiconductor structure 110 has been formed in an epitaxial growth process to include a group III-V semiconductor substrate contact part 112 and a group III-V semiconductor disc part 114.
  • the substrate contact part 112 is formed in the aperture 108.
  • the disc part 114 is formed above the insulating layer 102.
  • the substrate contact part 112 has a bottom portion 112a abutting the upper surface portion 100b.
  • the substrate contact part 112 has an upper portion 112b protruding from the aperture 108 above an upper surface 102c of the insulating layer 102.
  • the disc part 114 extends from the upper portion 112b of the substrate contact part 112, horizontally or laterally along the upper surface 102c (i.e.
  • the disc part 114 is accordingly formed to enclose the upper portion 112b of the substrate contact part 112 in a horizontal plane, or in other words enclose in a circumferential direction with respect to the substrate contact part 112. As may be seen in figures 7a and 7b a horizontal extension of the disc part 114 is such that the disc part 114 overlaps a portion 106a of the gate electrode 106.
  • the semiconductor structure 110 may be formed by vapor phase epitaxy.
  • a semiconductor structure 110 of any one of InSb, InAs, InGaAs or InGaSb may be epitaxially grown.
  • the process conditions of the epitaxial process may be controlled such that at least the at least the disc part 114 is formed of monocrystalline material, for instance monocrystalline InSb, InAs, InGaAs or InGaSb.
  • monocrystalline InSb, InAs, InGaAs or InGaSb for instance monocrystalline InSb, InAs, InGaAs or InGaSb.
  • the epitaxial growth process may be adapted such that, for the group III-V semiconductor disc part 114, a growth rate in a horizontal plane (i.e. along the first and second horizontal directions X and Y) is greater than a growth rate in a vertical direction Z.
  • the lateral/horizontal growth rate may for instance be (at least) a factor 10 greater than the vertical growth rate. This may be implemented during at least a part of the epitaxial growth process in which the disc part 114 is grown. However, as the lateral growth inside the aperture 108 will be limited by the lateral dimensions of the aperture 108 a lateral growth may be promoted throughout the epitaxial growth of the full semiconductor structure 110.
  • a lateral growth of a group III-V semiconductor material may be obtained by controlling a growth temperature to be in the range of 500 °C to 650 °C.
  • a total pressure (in the growth chamber) may be in the range of 20 mbar to 150 mbar.
  • the process conditions during the growth may be controlled such that a lateral growth rate in the range of 1 nm/s to 5 nm/s, and a vertical growth rate in or below the range 0.1 nm/s to 0.5 nm/s is obtained.
  • a mask 115 has been formed on the semiconductor structure 110.
  • the mask 115 may be referred to as the channel structure mask 115.
  • the mask 115 covers a portion of the disc part 114.
  • the masked portion may be formed as an elongated portion of the disc part 114, extending in the second horizontal direction Y.
  • the portion (as well as the mask 115) includes a sub-portion 114a extending across the portion 106a of the gate electrode 106.
  • the mask 115 may as shown in figure 9B be formed to expose the substrate contact part 112.
  • the mask 115 may be formed by depositing a mask layer on the semiconductor structure 110, such as a photo-resist based mask layer or any other typical lithographic stack compatible with the material of the semiconductor structure 110.
  • the mask 115 may subsequently be defined by patterning the mask layer using a conventional patterning technique.
  • the regions of the semiconductor structure 110 exposed by the mask 115 have been etched such that the masked portion of the disc part 114 remains to form a channel structure 116.
  • the channel structure 116 includes a portion 116a extending across the portion 106a of the gate electrode 106.
  • the channel structure 116 extends along the upper surface of the insulating layer 102 to be elongated in the second horizontal direction Y.
  • the mask 115 exposed the substrate contact part 112 the channel structure 116 is after completion of the etching disconnected from the substrate 100. Any remaining hole in the insulating layer 102 (i.e.
  • the hole previously exposing the portion 100b and accommodating the substrate contact part 112) may for instance be filled by an insulating material subsequent to the etch.
  • the semiconductor structure 110 may be etched using any conventional dry etching process allowing etching of the material of the semiconductor structure 110, such as a reactive ion etch (RIE) or ion beam etch (IBE). Subsequent to forming the channel structure 116 the mask 115 may be removed.
  • RIE reactive ion etch
  • IBE ion beam etch
  • a contact mask 118 (i.e. a source/drain contact mask 118) has been formed to cover the channel structure 116 and the insulating layer 102.
  • the contact mask 118 may for instance be a resist-based mask or a PMMA mask.
  • a source contact opening 118s exposing a source portion 116s of the channel structure 116 has been defined in the mask 118.
  • a drain contact opening 118d exposing a drain portion 116d of the channel structure 116 has been defined in the mask 118.
  • a superconductor source contact 120 and a superconductor drain contact 122 have been formed on the channel structure 116, at mutually opposite sides of the portion 106b of the gate electrode 106.
  • a superconductor has been deposited in the source contact opening 118s and in the drain contact opening 118d.
  • Al, Ta, Nd or Ti may be deposited by CVD, ALD or PVD.
  • the superconductor may be deposited to fill the openings 118s and 118d and cover the contact mask 118.
  • Overburden superconductor material may subsequently be removed from outside of the trench by CMP and/or an etch back process, thereby exposing an upper surface of the contact mask 118.
  • the contact mask 118 may subsequently wherein the contacts 120, 122 may remain on the channel structure 116.
  • the deposition of the superconductor in the contact openings 118s, 118d may be preceded by deposition of a superconductor seed layer (e.g. by selective area epitaxy) on portions of the channel structure 116 exposed in the contact openings 118s, 118d.
  • a gate contact mask 124 has been formed to cover the channel structure 116, the contacts 120, 122 and the insulating layer 102.
  • the gate contact mask 124 may for instance be a resist-based mask or a PMMA mask.
  • a pair of gate contact openings 124a, 124b have been defined in the mask 124, exposing respective portions of the insulating layer directly above the gate electrode 106.
  • a pair of gate contacts 126, 128 have been formed on mutually opposite sides of the channel structure 116.
  • the gate contact openings 124a, 124b have been transferred into the insulating layer 102 to form a pair of gate contact holes in the insulating layer 102.
  • the gate contact holes have subsequently been filled with a conductive contact material, for instance a metal such as a suitable CMOS-compatible metal contact material.
  • the contact material may be deposited by CVD, ALD or PVD, for instance.
  • Overburden contact material may subsequently be removed from outside of the gate contact holes by CMP and/or an etch back process, thereby exposing an upper surface of the gate contact mask 124.
  • the mask 124 has subsequently been removed wherein the contacts 126, 128 remain on the gate electrode 106.
  • the resulting device has a configuration which makes it suitable for use as a qubit device which, under the correct operating conditions as is known in the art, enables forming of Majorana fermions and conduction of the same along the channel structure 116 between the superconductor source and drain electrodes 120, 122.
  • the configuration of the gate electrode 106 allows a chemical potential in the nanostructure 116 to be adapted to enable the Majorana fermions, i.e. qubits, to be manipulated.
  • a plurality of such qubit may be formed on the substrate 100 and interconnected to each other to implement qubit logic gates.
  • the channel structures may be formed to extend across a respective portion 106a of the gate electrode 106.
  • the channel structures may be formed to extend in parallel to each other.
  • More than one channel structure may be formed by patterning the channel structure mask (corresponding to mask 115 above) to include a number of discrete mask portions, each defining a respective channel structure.
  • figure 14 illustrates a qubit device wherein additional embedded gate electrodes 206, 306, parallel to the gate electrode 106, have been formed along the channel structure 116.
  • a respective further pair of gate contacts 226, 228 and 326, 328 have been formed on the gate contacts 206, 306.
  • the further gate electrodes 206, 306 may be formed by forming further trenches parallel to the trench 104, as disclosed in connection with figures 3A and 3B .
  • Three gate electrodes represents however only one example and a qubit device may be formed to include any number of embedded gate electrodes along the channel structure 116, however at least one.

Description

    Technical field
  • The present inventive concept relates to a method for forming a qubit device. The present inventive concept further relates to a qubit device.
    • Background
  • Quantum information science has the potential to radically improve existing techniques and devices for sensing, computation, simulation, and communication.
  • A major challenge in quantum information processing systems is to realize qubits with a sufficient degree of coherence while still allowing manipulation and measurements. One type of qubit device showing promise are devices based on so-called Majorana fermions. A Majorana fermion is its own antiparticle and may in a quantum mechanical framework be described as a superposition of an electron and a hole.
  • Although qubit devices based on Majorana fermions have been demonstrated on a lab-scale, realizing industrially viable and scalable qubit devices remains a challenge. Therefore, quantum information processing experiments are still expensive and time consuming endeavors. Existing laboratory setups may also be challenging to fine tune and scale.
  • WO 2016/000836 A1 discloses semiconductor based Josephson junctions and their applications within the field of quantum computing, in particular a tuneable Josephson junction device being used to construct a gateable transmon qubit. The reference discloses a Josephson junction comprising an elongated hybrid nanostructure comprising superconductor and semiconductor materials and a weak link, wherein the semiconductor weak link is formed by a semiconductor segment of the elongated hybrid nanostructure where the superconductor material has been removed. An elongated hybrid nanostructure is liberated from their growth substrate by a brief sonication and a small amount of the resulting suspension can subsequently be deposited on doped Si substrates capped with 500 nm SiO2.
  • V. Mourik et al reports in "Signatures of Majorana Fermions in Hybrid Superconductor-Semiconductor Nanowire Devices" (SCIENCE, vol. 336, no. 6084, 25 May 2012, pages 1003-1007, ISSN: 0036-8075, DOI: 10.1126/science. 1222360) of electrical measurements on indium antimonide nanowires contacted with one normal (gold) and one superconducting (niobium titanium nitride) electrode. It is stated the observations support the hypothesis of Majorana fermions in nanowires coupled to superconductors.
  • Roddare S et al discloses in "InAs nanowire hot-electron Josephson transistor" (ARXIV.ORG, CORNELL UNIVERSITY LIBRARY, 201 OLIN LIBRARY CORNELL UNIVERSITY ITHACA, NY 14853, 10 March 2010, DOI: 10.1007/S12274-010-0077-6) an InAs NW Josephson transistor where supercurrent is controlled by hot-quasiparticle injection from normalmetal electrodes.
  • Leonik P. Rokhinson et al reports in "The fractional a.c. Josephson effect in a semiconductor-superconductor nanowire as a signature of Majorana particles" (NATURE PHYSICS, vol. 8, no. 11, 23 September 2012, pages 795-799, ISSN: 1745-2473, DOI: 10.1038/nphys2429) observations of the fractional a.c. Josephson effect in a hybrid semiconductor-superconductor InSb/Nb nanowire junction.
    • Summary
  • An objective of the present inventive concept is to address this issue in the prior art. Further and alternative objectives may be understood from the following.
  • According to an aspect of the present inventive concept there is provided a method for forming a qubit device according to claim 1.
  • The inventive method enables accurate, repeatable and scalable manufacturing of qubit devices. The combination of a group III-V semiconductor channel structure and superconductor source and drain contacts enable Majorana fermion based qubit devices. By qubit device is hereby meant a semiconductor device with a configuration such that, under the correct operating conditions, it may support or provide states or particles (or more specifically quasiparticles in the case of Majorana fermions) which may operate as qubits whose state may be manipulated and detected.
  • As the channel structure is formed by patterning an epitaxially grown group III-V material structure the typical prior art approach of growing nanostructures (such as vertical nanowires) on a separate wafer and thereafter transferring them to a target substrate may be avoided. The method thereby allows an overall more efficient process as transfer of nanowires to intended positions at a target substrate may be delicate and difficult to scale.
  • Furthermore, the forming of the channel structure by patterning allows a considerable degree of control of the dimension, shape and arrangement of the channel structure in relation to the gate electrode.
  • By forming the gate electrode as an embedded gate electrode, exposure of the channel structure to the potentially adverse process conditions of gate electrode and gate dielectric formation may be avoided. Moreover, the embedded configuration of the gate electrode enables a gate-channel coupling of a strength suitable for qubit device operation to be achieved.
  • In terms of device properties, the method enables scalable fabrication of qubit devices with an improved uniformity in terms of structure and performance.
  • The superconductor source contact and the superconductor drain contact are advantageously formed subsequent to forming the channel structure. More specifically, the method may comprise forming the superconductor source contact and the superconductor drain contact on the channel structure at mutually opposite sides of said portion of the gate electrode. This facilitates achieving a proper alignment between the source/drain portions of the channel structure and the source/drain contacts.
  • By the upper surface of the substrate is intended a main surface of the substrate on which the processing according to the inventive method is performed. Hence, "upper" should not be construed to require a particular orientation of the substrate.
  • As used herein, the term first horizontal direction refers to a direction along the substrate, i.e. along the upper surface of the substrate. Correspondingly, the second horizontal direction refers to a direction along the substrate, which second horizontal direction is different from the first horizontal direction. The first and second horizontal directions may represent mutually perpendicular directions but may more generally extend at an angle with respect to each other. Accordingly, the term horizontal plane may be used to refer to a plane defined by the first and second horizontal directions. In other words, the term horizontal plane may denote a plane parallel to the upper (main) surface of the substrate.
  • A vertical direction may refer to a direction which is normal to the upper (main) surface of the substrate, or perpendicular to the first and second horizontal directions.
  • By a superconductor is hereby meant any material exhibiting superconductivity when kept at a temperature below a critical temperature Tc, specific to the material. By way of example, Tc of Al is 1.20 K, Tc of Ta is 4.48 K and TC of Ti is 0.39 K, TC of Nd is 9.26 K.
  • According to the invention, the mask is formed such that the substrate contact part is exposed by the mask. Accordingly the substrate contact part is removed during the etching wherein the channel structure is electrically insulated from the substrate by the insulator layer.
  • According to one embodiment, said forming of the gate electrode embedded in the insulating layer comprises:
    • forming a first partial insulating layer on the upper surface of the substrate,
    • forming a trench extending partially through the first partial insulating layer,
    • forming the gate electrode in the trench, and
    • covering the gate electrode and the first partial insulating layer with a second partial insulating layer, wherein the first partial insulating layer and the second partial insulating layer together forms said insulating layer. As the trench is formed to extend only partially through the insulating layer, the gate electrode may be formed on an electrically insulating bottom surface of the trench.
  • According to the invention, the gate electrode is formed to be elongated in the first horizontal direction. An elongated gate electrode enables definition of a short gate-channel interface, along the direction of the channel. A highly localized gate control may thereby be achieved.
  • The method may further comprise forming a pair of gate contacts on the gate electrode, at mutually opposite sides of the channel structure . Easy accessible electrical contact to the gate electrode may thereby be provided. In case the gate electrode is formed to be elongated the contacts may be formed spaced apart from the channel structure to mitigate undesired stray capacitive coupling between the contacts and the channel.
  • According to one embodiment said gate electrode forms a first gate electrode and the method further comprises forming at least a second gate electrode, each one of the gate electrodes being embedded in the insulating layer and extending in said first direction. Providing more than gate electrode enables improved channel control along the length of the channel structure.
  • The semiconductor disc part may be formed to overlap a respective portion of each one of the gate electrodes, wherein the channel structure may extend across each one of said portions of the gate electrodes. Gate electrode-channel structure overlap may thereby be obtained at plural positions in an efficient manner.
  • The epitaxial growth process may be adapted such that a growth rate in a horizontal plane is greater than a growth rate in a vertical direction during a part of the epitaxial growth process in which the disc part is grown. Thereby a disc part having a greater horizontal dimension than vertical dimension may be grown. This in turn enables forming of a relatively long and thin channel structure.
  • According to the invention the channel structure is formed to be elongated in the second horizontal direction, the second horizontal direction being different from the first horizontal direction.
  • Combined with an epitaxial growth process promoting a horizontal plane growth rate a channel structure having a high length to thickness ratio and, optionally, a high width to thickness ratio may be achieved. Such a channel structure may be referred to as a nanowire.
  • The disc part may be formed to enclose the upper portion of the substrate contact part in a horizontal plane. The channel structure may hence be formed to extend on either side of the substrate contact part.
  • According to one embodiment the forming of the source and drain superconductor contacts may comprise:
    • forming a contact mask including a source contact opening exposing a source portion of the channel structure, and further including a drain contact opening exposing a drain portion of the channel structure, and
    • forming the source superconductor contact in the source contact opening and the drain superconductor contact in the drain contact opening.
  • Superconductor contacts may thereby be conveniently formed wherein portions of the channel structure covered by the contact mask (i.e. portions other than the source portion and the drain portion) may be protected, at least to some extent, from the process conditions of the contact formation. The forming of the source and drain superconductor contacts in the respective mask openings may include deposition of a superconductor material in the respective openings. Optionally, a superconductor seed layer may be formed on portions of the channel structure exposed in the contact openings. This may improve an interface quality between the superconductor source/drain contacts and the channel structure.
  • The superconductor source contact and the superconductor drain contact may include, or consist of, Al, Ta, Ti or Nd.
  • The epitaxial growth process may comprise growing the semiconductor structure of InSb, InAs, InGaAs or InGaSb. These semiconductor materials, when used in combination with superconductor contacts, enable formation of Majorana fermions in the device channel.
  • Advantageously, to enable improved device operation, at least the disc part may be formed of monocrystalline InSb, InAs, InGaAs or InGaSb.
    • Brief description of the drawings
  • The above, as well as additional objects, features and advantages of the present inventive concept, will be better understood through the following illustrative and non-limiting detailed description, with reference to the appended drawings. In the drawings like reference numerals will be used for like elements unless stated otherwise.
    • Figures 1-13 schematically illustrate a method for forming a qubit device.
    • Figure 14 schematically shows a variation of a qubit device.
    Detailed description
  • A method for forming a qubit device will now be disclosed in connection with figures 1-13. Each one of the figures shows a cross-sectional side view (figure a) of the structure and a top down view (figure b). The cross-sectional view in figure A is taken along the geometrical line indicated in the correspondingly numbered figure B (i.e. line AA' or BB'). In the figures the axis Z denotes a vertical direction, corresponding to a normal direction with respect to an upper surface 100a of a substrate 100. The axes X and Y refer to mutually orthogonal first and second horizontal directions, i.e. directions being parallel to the main plane of extension of the substrate 100 (or correspondingly the upper surface 100a thereof). It should be noted that the relative dimensions of the shown elements, such as the relative thickness of the layers of structures, is merely schematic and may, for the purpose of illustrational clarity, differ from a physical structure.
  • Figures 1A, 1B illustrate a semiconductor substrate 100. The semiconductor substrate 100 may be single material substrate or be formed of a combination of different materials, such as of various layers stacked on top of each other. In any case at least an upper surface 100a of the substrate 100 is formed of an elemental group IV semiconductor or compound group IV semiconductor. The upper surface may be formed by a [111] face of silicon (Si). Possible substrate 100 structures include a Si substrate or a silicon-on-insulator (SOI) substrate. The upper surface 100a of the substrate 100 may be referred to as a front side surface of the substrate 100.
  • In figures 2A, 2B a first partial insulating layer 102a has been formed on the upper surface 100a of the substrate 100. The first partial insulating layer 102a covers the upper surface 100a. The first partial insulating layer 102a may be an oxide layer, for instance a silicon oxide such as SiO2. The first partial insulating layer 102a may also be a dielectric layer of for instance a high-K dielectric material such as aluminum oxide or some other CMOS compatible gate dielectric. The first partial insulating layer 102a may be deposited on the surface 100a by any suitable and conventional deposition technique, such as chemical vapor deposition (CVD) or atomic layer deposition (ALD), or by thermal oxidation.
  • In figures 3A, 3B a trench 104 extending partially through the first partial insulating layer 102a has been formed. A longitudinal dimension of the trench 104 extends in the first horizontal direction X. A vertical dimension of the trench 104 extends in the vertical direction Z. As indicated in figures 3A, 3B a bottom surface of the trench 104 is formed by a remaining thickness portion of the first partial insulating layer 102a. A trench mask layer 103 has been formed on the first partial insulating layer 102a. A trench defining opening has been formed in the mask layer 103. The opening has subsequently been transferred into the first partial insulating layer 102a by etching of the material of the first partial insulating layer 102a to form the trench 104. The mask layer 103 may be a photo-resist based mask layer 103 of any other typical lithographic stack compatible with the material forming the first partial insulating layer 102a. Any conventional wet or dry etching process allowing etching of the material of the first partial insulating layer 102a may be employed. Subsequent to etching the trench 104, the mask layer 103 may be removed. The trench 104 may by way of example be formed with a width in the range of 20-100 nm, depending among others on the intended length of the final channel structure which is to be formed.
  • In figures 4A, 4B, a gate electrode 106 has been formed in the trench 104. The gate electrode 106 is elongated and extends along the substrate upper surface, in the first horizontal direction X. The gate electrode 106 is electrically insulated from the substrate 100 by the insulating layer 102. A conductive gate electrode material may be deposited in the trench 104. The conductive gate electrode material may be a metal or a metal alloy. However other gate electrode materials are also possible, such as polysilicon. The material may be deposited by any conventional deposition technique, such as by ALD, CVD or physical vapor deposition (PVD). The material may be deposited to fill the trench 104 and cover the first partial insulating layer 102a. Overburden gate electrode material (i.e. material portions deposited outside of the trench 104) may subsequently be removed from outside of the trench by chemical mechanical polishing (CMP) and/or an etch back process, thereby exposing an upper surface of the insulating layer 102a.
  • In figures 5A, 5B, the gate electrode 106 and the first partial insulating layer 102a has been covered by a second partial insulating layer 102b. The first partial insulating layer 102a and the second partial insulating layer 102b together form a compound insulating layer, hereinafter referred to as the insulating layer 102, embedding the gate electrode 106. The second partial insulating layer 102b may be formed of a same material as the first partial insulating layer 102a. The second partial insulating layer 102b may be deposited in a similar manner as the first partial insulating layer 102a. A total thickness of the insulating layer 102 may for instance be in the range of 10-50 nm.
  • In figures 6A, 6B, an aperture 108 has been formed in the insulating layer 102. The aperture 108 exposes an upper surface portion 100b (i.e. a portion of the upper surface 100a) of the substrate 100. The aperture 108 extends vertically through the insulating layer 102. An aperture mask layer 107 has been formed on the insulating layer 102. An aperture defining opening has been formed in the mask layer 107. The opening has subsequently been transferred into the insulating layer 102 by etching of the material (of the insulating layer 102 to form the aperture 108. The mask layer 107 may be a photo-resist based mask layer 107 of any other typical lithographic stack compatible with the material forming the first partial insulating layer 102a. Any conventional wet or dry etching process allowing etching of the material of the insulating layer 102 may be employed. Subsequent to etching the aperture 108 the mask layer 107 may be removed.
  • In figures 7A, 7B, a semiconductor structure 110 has been formed in an epitaxial growth process to include a group III-V semiconductor substrate contact part 112 and a group III-V semiconductor disc part 114. The substrate contact part 112 is formed in the aperture 108. The disc part 114 is formed above the insulating layer 102. The substrate contact part 112 has a bottom portion 112a abutting the upper surface portion 100b. The substrate contact part 112 has an upper portion 112b protruding from the aperture 108 above an upper surface 102c of the insulating layer 102. The disc part 114 extends from the upper portion 112b of the substrate contact part 112, horizontally or laterally along the upper surface 102c (i.e. in the horizontal plane defined by the axes X and Y). The disc part 114 is accordingly formed to enclose the upper portion 112b of the substrate contact part 112 in a horizontal plane, or in other words enclose in a circumferential direction with respect to the substrate contact part 112. As may be seen in figures 7a and 7b a horizontal extension of the disc part 114 is such that the disc part 114 overlaps a portion 106a of the gate electrode 106. The semiconductor structure 110 may be formed by vapor phase epitaxy.
  • It should be noted that the particular polygonal shape of the disc part 114 illustrated in figure 7b merely is one example and that other shapes, such as generally hexagonal shapes, also are possible. The specific shape typically may depend on the growth conditions and the orientation of the growth surfaces.
  • A semiconductor structure 110 of any one of InSb, InAs, InGaAs or InGaSb may be epitaxially grown. The process conditions of the epitaxial process may be controlled such that at least the at least the disc part 114 is formed of monocrystalline material, for instance monocrystalline InSb, InAs, InGaAs or InGaSb. However, it may also be possible to use other materials with a sufficiently large Lande G-factor.
  • The epitaxial growth process may be adapted such that, for the group III-V semiconductor disc part 114, a growth rate in a horizontal plane (i.e. along the first and second horizontal directions X and Y) is greater than a growth rate in a vertical direction Z. The lateral/horizontal growth rate may for instance be (at least) a factor 10 greater than the vertical growth rate. This may be implemented during at least a part of the epitaxial growth process in which the disc part 114 is grown. However, as the lateral growth inside the aperture 108 will be limited by the lateral dimensions of the aperture 108 a lateral growth may be promoted throughout the epitaxial growth of the full semiconductor structure 110.
  • For instance, a lateral growth of a group III-V semiconductor material may be obtained by controlling a growth temperature to be in the range of 500 °C to 650 °C. A total pressure (in the growth chamber) may be in the range of 20 mbar to 150 mbar. The process conditions during the growth may be controlled such that a lateral growth rate in the range of 1 nm/s to 5 nm/s, and a vertical growth rate in or below the range 0.1 nm/s to 0.5 nm/s is obtained.
  • In figures 8A, 8B a mask 115 has been formed on the semiconductor structure 110. The mask 115 may be referred to as the channel structure mask 115. The mask 115 covers a portion of the disc part 114. The masked portion may be formed as an elongated portion of the disc part 114, extending in the second horizontal direction Y. The portion (as well as the mask 115) includes a sub-portion 114a extending across the portion 106a of the gate electrode 106. The mask 115 may as shown in figure 9B be formed to expose the substrate contact part 112. The mask 115 may be formed by depositing a mask layer on the semiconductor structure 110, such as a photo-resist based mask layer or any other typical lithographic stack compatible with the material of the semiconductor structure 110. The mask 115 may subsequently be defined by patterning the mask layer using a conventional patterning technique.
  • In figures 9A, 9B the regions of the semiconductor structure 110 exposed by the mask 115 have been etched such that the masked portion of the disc part 114 remains to form a channel structure 116. As shown in figures 9A, 9B the channel structure 116 includes a portion 116a extending across the portion 106a of the gate electrode 106. The channel structure 116 extends along the upper surface of the insulating layer 102 to be elongated in the second horizontal direction Y. As the mask 115 exposed the substrate contact part 112 the channel structure 116 is after completion of the etching disconnected from the substrate 100. Any remaining hole in the insulating layer 102 (i.e. the hole previously exposing the portion 100b and accommodating the substrate contact part 112) may for instance be filled by an insulating material subsequent to the etch. The semiconductor structure 110 may be etched using any conventional dry etching process allowing etching of the material of the semiconductor structure 110, such as a reactive ion etch (RIE) or ion beam etch (IBE). Subsequent to forming the channel structure 116 the mask 115 may be removed.
  • In figures 10A, 10B a contact mask 118 (i.e. a source/drain contact mask 118) has been formed to cover the channel structure 116 and the insulating layer 102. The contact mask 118 may for instance be a resist-based mask or a PMMA mask. A source contact opening 118s exposing a source portion 116s of the channel structure 116 has been defined in the mask 118. Further, a drain contact opening 118d exposing a drain portion 116d of the channel structure 116 has been defined in the mask 118.
  • In figures 11A, 11B a superconductor source contact 120 and a superconductor drain contact 122 have been formed on the channel structure 116, at mutually opposite sides of the portion 106b of the gate electrode 106. A superconductor has been deposited in the source contact opening 118s and in the drain contact opening 118d. For instance Al, Ta, Nd or Ti may be deposited by CVD, ALD or PVD. The superconductor may be deposited to fill the openings 118s and 118d and cover the contact mask 118. Overburden superconductor material may subsequently be removed from outside of the trench by CMP and/or an etch back process, thereby exposing an upper surface of the contact mask 118. The contact mask 118 may subsequently wherein the contacts 120, 122 may remain on the channel structure 116. Optionally, the deposition of the superconductor in the contact openings 118s, 118d may be preceded by deposition of a superconductor seed layer (e.g. by selective area epitaxy) on portions of the channel structure 116 exposed in the contact openings 118s, 118d.
  • In figures 12A, 12B a gate contact mask 124 has been formed to cover the channel structure 116, the contacts 120, 122 and the insulating layer 102. The gate contact mask 124 may for instance be a resist-based mask or a PMMA mask. A pair of gate contact openings 124a, 124b have been defined in the mask 124, exposing respective portions of the insulating layer directly above the gate electrode 106.
  • In figures 13A, 13B, a pair of gate contacts 126, 128 have been formed on mutually opposite sides of the channel structure 116. The gate contact openings 124a, 124b have been transferred into the insulating layer 102 to form a pair of gate contact holes in the insulating layer 102. The gate contact holes have subsequently been filled with a conductive contact material, for instance a metal such as a suitable CMOS-compatible metal contact material. The contact material may be deposited by CVD, ALD or PVD, for instance. Overburden contact material may subsequently be removed from outside of the gate contact holes by CMP and/or an etch back process, thereby exposing an upper surface of the gate contact mask 124. The mask 124 has subsequently been removed wherein the contacts 126, 128 remain on the gate electrode 106.
  • The resulting device has a configuration which makes it suitable for use as a qubit device which, under the correct operating conditions as is known in the art, enables forming of Majorana fermions and conduction of the same along the channel structure 116 between the superconductor source and drain electrodes 120, 122. The configuration of the gate electrode 106 allows a chemical potential in the nanostructure 116 to be adapted to enable the Majorana fermions, i.e. qubits, to be manipulated. Advantageously, a plurality of such qubit may be formed on the substrate 100 and interconnected to each other to implement qubit logic gates.
  • In the above the inventive concept has mainly been described with reference to a limited number of examples. However, as is readily appreciated by a person skilled in the art, other examples than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.
  • For instance, it is possible to form more than one, such as two or more, channel structures simultaneously from the disc part 114. The channel structures may be formed to extend across a respective portion 106a of the gate electrode 106. The channel structures may be formed to extend in parallel to each other. More than one channel structure may be formed by patterning the channel structure mask (corresponding to mask 115 above) to include a number of discrete mask portions, each defining a respective channel structure.
  • According to a further variation, figure 14 illustrates a qubit device wherein additional embedded gate electrodes 206, 306, parallel to the gate electrode 106, have been formed along the channel structure 116. A respective further pair of gate contacts 226, 228 and 326, 328 have been formed on the gate contacts 206, 306. The further gate electrodes 206, 306 may be formed by forming further trenches parallel to the trench 104, as disclosed in connection with figures 3A and 3B. Three gate electrodes represents however only one example and a qubit device may be formed to include any number of embedded gate electrodes along the channel structure 116, however at least one.

Claims (11)

  1. A method for forming a qubit device, the method comprising:
    forming a gate electrode (106) embedded in an insulating layer (102) formed on a substrate (100), wherein an upper surface (100a) of the substrate (100) is formed by a group IV semiconductor material and wherein the gate electrode (106) extends along the substrate (100) and is formed to be elongated in a first horizontal direction,
    forming an aperture (108) in the insulating layer (102), the aperture (108) exposing a portion (100b) of the substrate (100),
    forming in an epitaxial growth process, a semiconductor structure (110) comprising a group III-V semiconductor substrate contact part (112) and a group III-V semiconductor disc part (114),
    the substrate contact part (112) having a bottom portion (112a) abutting said exposed portion (100b) of the substrate (100) and an upper portion (112b) protruding from the aperture (108) above an upper surface (102c) of the insulating layer (102),
    the semiconductor disc part (114) extending from said upper portion (112b) of the substrate contact part (112), horizontally along said upper surface (102c) of the insulating layer (102) to overlap a portion (106a) of the gate electrode (106),
    forming a mask (115) covering a portion of the semiconductor disc part (114), said covered portion of the semiconductor disc part (114) being elongated and extending across said portion (106a) of the gate electrode (106) in a second horizontal direction, different from said first horizontal direction, wherein the substrate contact part (112) is exposed by said mask (115),
    etching the regions of the semiconductor structure (110) exposed by the mask (115) such that the masked portion of the semiconductor disc part (114) remains to form a channel structure (116) elongated in said second horizontal direction and extending across said portion (106a) of the gate electrode (106), and
    forming a superconductor source contact (120) and a superconductor drain contact (122) on the channel structure (116) at mutually opposite sides of said portion (106b) of the gate electrode (106).
  2. A method according to claim 1, said forming of the gate electrode (106) embedded in the insulating layer (102) comprises:
    forming a first partial insulating layer (102a) on the upper surface (100a) of the substrate (100),
    forming a trench (104) extending partially through the first partial insulating layer (102a),
    forming the gate electrode (106) in the trench (104), and
    covering the gate electrode (106) and the first partial insulating layer (102a) with a second partial insulating layer (102b), wherein the first partial insulating layer (102a) and the second partial insulating layer (102b) together form said insulating layer (102).
  3. A method according to any of the preceding claims, further comprising forming a pair of gate contacts (126, 128) on the gate electrode (106), at mutually opposite sides of the channel structure (116).
  4. A method according to any of claims 2-3, wherein said gate electrode (106) forms a first gate electrode (106) and at least a second gate electrode (206), each gate electrode (106, 206) being embedded in the insulating layer (102) and extending in said first horizontal direction.
  5. A method according to claim 4, wherein the semiconductor disc part (114) is formed to overlap a respective portion of each one of the gate electrodes (106, 206), and wherein the channel structure (116) extends across each one of said portions of the gate electrodes (106, 206).
  6. A method according to any of the preceding claims, wherein the epitaxial growth process is adapted such that a growth rate in a horizontal plane is greater than a growth rate in a vertical direction during a part of the epitaxial growth process in which the disc part (114) is grown.
  7. A method according to any of the preceding claims, wherein the disc part (114) is formed to enclose the upper portion (112b) of the substrate contact part (112) in a horizontal plane.
  8. A method according to any of the preceding claims, wherein the forming of the source and drain superconductor contacts (120, 122) comprises:
    forming a contact mask (118) including a source contact opening (118s) exposing a source portion (116s) of the channel structure (116), and further including a drain contact opening (118d) exposing a drain portion (116d) of the channel structure (116), and
    forming the source superconductor contact in the source contact opening (118s) and the drain superconductor contact in the drain contact opening (118d).
  9. A method according to any of the preceding claims, wherein the superconductor source contact (120) and the superconductor drain contact (122) includes Al, Ta, Ti or Nd.
  10. A method according to any of the preceding claims, wherein the epitaxial growth process comprises growing the semiconductor structure (110) of InSb, InAs, InGaAs or InGaSb.
  11. A method according to claim 10, wherein at least the disc part (114) is formed of monocrystalline InSb, InAs, InGaAs or InGaSb.
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